Current laser printers employ cyan (C), magenta (M), yellow (Y) and black (K) toners to produce full color images. Theoretically, a mixture of CMY, in equal amounts should produce black, just as an additive combination of red, green and blue colors should produce white. However, actual printing with CMY toners generally does not produce a true black, but instead provides a muddy, grayish-brown. This is due to impurities in the toners, reflection of light from the surface of the toner (so that it does not pass through the toner layers to be absorbed) and difficulty in obtaining complete coverage of white paper.
The most common solution is to add a separate black toner to the printing process. This reduces the need for large amounts of the color toners, reduces the thickness of ink build-up on the page and reduces cost. But, from the standpoint of image quality, the most important factor is that it allows dark colors to be printed, without appearing muddy.
The CMYK system employs four half-tone screens, one for each toner. However, while converting from red, green, blue colors to CMY (often called process color) is straightforward, converting to CMYK is not. Rules to calculate how much black to put into various color representations depend far more on visual response to colors, the kind of paper to be printed, the illumination with which the print will be viewed and even the contents of the images.
Algorithms for converting CMY to CMYK involve specifying levels of undercolor removal (UCR) and grey component replacement (GCR). One approach is to use the value of whichever of the three components (CMY) is least intense to determine an amount of black to be added. For instance, for a color containing 80% cyan, 50% magenta and 30% yellow, the 30% value is taken as an index into a built-in calibration curve. This might indicate that a 15% value for the black toner should chosen for GCR. Then, the amount of the principal color (in this example, C) or the amounts of all of the colors are reduced. In the latter case, the 30% value is known as the "Min" function which essentially defines the minimum value of all of the constituent colors at the specific location in the image.
The UCR and GCR processes can consume considerable memory during the processing of an image. UCR procedures require copies of C, M, Y and K color planes to be retained while the UCR processing is in progress. As reduction of memory utilization in a printer directly equates to lowered printer costs, continuing efforts are being made to improve memory utilization during high usage operations, such as during image rasterization and UCR actions.
Accordingly, it is an object of this invention to provide an improved method and apparatus for enabling UCR in a full color printer.
It is an another object of this invention to provide an method and apparatus for UCR wherein memory utilization is improved.